This invention relates generally to apparatus for producing bituminous paving mixtures, such as asphalt concrete, and more particularly concerns a closed loop system for producing such mixtures.
Bituminous paving mixtures are produced by combining selected proportions of sized or graded aggregate with a bituminous binder in the presence of relatively high heat. These mixtures, which flow when produced, harden upon cooling to form pavement. In early apparatus, the aggregates were first heated to remove any residual moisture, but new devices, referred to as drum dryer mixers, or just drum mixers, mix unconditioned aggregates and the bituminous binder. The drum mixer has numerous advantages over the older apparatus and has achieved a significant degree of acceptance in the industry. An example of such an apparatus is disclosed in U.S. Pat. No. 3,832,201, issued Aug. 27, 1974, to H. N. Shearer, titled "Process For Making Asphalt Paving Compositions."
A primary concern in the asphalt concrete industry, particularly over the past several years, is the pollution caused by the exhaust of particulates from asphalt plants or drum mixer operations. The particulates or "fines" include the fine aggregates which become airborne in the drum mixer or when the aggregates are being dried, and which are never captured in the mixing process. Particulate volumes as much as 3000 Lbs/hour are not uncommon in the industry, requiring complex systems of pollution control devices downstream of the mixer.
The '201 patent discloses a particular drum mixer arrangement, however, which reduces the amount of exhaust particulates to the point where pollution control equipment is virtually unnecessary. In that particular drum mixer, the graded aggregates and the bituminous binder are added together at the intake end of drum mixer, so that the binder, and later the mixture, captures nearly all the small particulates or fines which would otherwise become airborne and be exhausted from the apparatus.
Although such arrangement substantially reduces the volume of particulates exhausted to the atmosphere, certain disadvantages resulted, due to the proximity of the burner flame to the region of the intake of the bituminous binder. The presence of the flame, resulting in core temperatures as high as 3000.degree. F. in the intake region, and a high heat transfer rate, caused combustion of the bituminous binder and a resulting dense smoke at the exhaust, as well as a decrease in the quality of the mix. In addition, the high heat transfer rates of such devices prevented the recycling of existing bituminous pavement, as such material smokes in such drum mixers when added at the intake region.
In certain drum mixers, the bituminous binder is added downstream of the intake region, which eliminates the combustion of the binder but brings the particulate level up significantly. Other devices, such as those disclosed in U.S. Pat. No. 4,039,171 to H. N. Shearer, titled "Drum Dryer/Mixer", and in U.S. patent application No. 879,407, now U.S. Pat. No. 4,143,972, in the name of Bernard A. Benson, filed Feb. 21, 1978 titled: "Combustion Control System for Bituminous Drum Mixers," both of which are assigned to the assignee of the present invention, attempt to solve this problem. In the mixers disclosed therein, screens or shields are used to modify the thermal effects of the burner flame to the point where the bituminous binder may be added to the intake region. These techniques themselves have disadvantages, however. The screens have a short life, due to the high temperature of the flame, and the shield likewise is not completely satisfactory, again due to the immediate proximity of the burner flame.
In addition, such apparatus for producing bituminous paving mixtures use a great deal of fuel to produce the high temperatures required, and to date, the high temperature gases present at the discharge end of the apparature are exhausted to the atmosphere. A great deal of energy, in the form of heated exhaust gas, is thus lost to the atmosphere, which reduces the overall thermal efficiency of the system. The inefficient use of energy, of course, is becoming increasingly important in this time of scarce resources.
Accordingly, it is a general object of the present invention to provide a system for producing bituminous paving mixtures which overcomes one or more of the disadvantages of the prior art specified above.
It is another object of the present invention to provide such a system which permits mixing of graded aggregates and bituminous binders at the intake end of a drum mixer portion of the system, without resulting in smoking of the binder.
It is a further object of the present invention to provide such a system which has an increased thermal efficiency over existing systems.
It is an additional object of the present invention to provide such a system which does not require additional pollution equipment.
It is a still further object of the present invention to provide such a system in which the heat in the intake region of the drum mixer is substantially equal across the drum mixer.
It is yet another object of the present invention to provide such a system which is capable of recycling bituminous paving mixtures.